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THE TRANSPORTATION OF GASES                        173
111., shows the variation in capacity and horsepower for various altitudes'. The altitudes given are heights above mean sea level and are subject to correction for temperature and latitude. From the table it can be seen that for a two-stage compressor discharging at 100 Ib. pressure when operating at an altitude of 8,000 ft. the volumetric capacity will be only 76 per cent of that at mean sea level, while the horsepower required will be 85 per cent of that at mean sea level.
Unloading Devices.—Many compressors operate at constant speed, independent of the demands for compressed air; and, in order to secure economy of operation for this condition, various types of "unloaders" have been designed, For small single-stage compressors the Sullivan Machinery Co. provides an un-
Mecxn      Effective     Pressures
Full Load                   Is-* Step.                2n^5tep.                 3^ Step.                VS Step.
FIG. 33.—Cards from high-pressure cylinder of a two-stage compressor with clearance
loading valve connected by piping to the inlet valves. When the predetermined pressure is exceeded, the unloader raises the inlet valves from their seats and prevents further compression of air until the pressure falls a few pounds, when the unloader allows the valves to resume their seats and the work of compression is again taken up. Other manufacturers use a similar device to keep the inlet closed when the predetermined pressure is reached. For larger compressors a double-beat valve is used, which is placed on the air inlet duct and controlled by air pressure from the air receiver. This valve is set to shut off from the compressor all or part of the incoming air when the receiver pressure rises above predetermined point.
When the speed is not constant other types of regulating devices are available. For simple belt-driven compressors a belt shifter can be designed to shift the belt from the tight to the loose pulley when the predetermined air pressure is reached, but for steam-driven compressors the device is usually one for governing the speed of the compressor to suit the demands for compressed air.
Water Jackets, Intercoolers, Receivers and Aftercoolers.—The cooling surface in an intercooler is generally designed from the formula 3 = Q/Q.25(ta — tv), where S is the cooling surface in square feet, Q is the number of cubic feet of free air per minute, and ta and tw are the temperatures, in degrees F. of the air leaving and the water entering the intercooler, respectively.
The amount of cooling water required is given by Longacre as G = 75 + 2.5£ when the intercooler and jacket are in series; G = 20 + 2t for a separate intercooler; and G = 2t for low-pressure and high-pressure jackets only. In these formulas G is in gallons of water per hour per 100 cu. ft. of free air per minute and t is the entering temperature, degrees F. of the cooling water.
Receivers are used to supply a reservoir of air; to equalize the pulsations in the air coming from the compressor; to collect the water and grease held in suspension by the compressed air as it leaves the compressor; to reduce the friction of air in the pipe system; and to cool the air as thoroughly as possible before entering the transmission system. To facilitate the removal of water from the